The zoom select tool is a quick way to zoom into an area of interest. If you click the zoom select tool and draw your selection box the selected area will expand to fit the selection box. The drawn selection box will expand to fit your Workspace so that the magnification will will vary according to the size of the selection box and the area of the model selected.
Alternatively you can always have zoom select zoom to 100% by clicking the zoom select tool then holding the shift key down while you draw the selection box and release the mouse key.